Hydronic heating systems can provide comfort for your home, like this one in the floor.

Hydronic heating systems can provide comfort for your home, like this one on the wall.

The Heating Edge is a recent development in low-temperature fin-tube baseboard heating (detail).

The Heating Edge is a recent development in low-temperature fin-tube baseboard heating.

Tubing embedded in a concrete floor slab is the most common form of radiant floor heating.

A thin-slab radiant panel installation awaits the concrete pour. The 1/2-inch PEX-AL-PEX tubing has been carefully fastened using a special stapler. A layer of 6-mil polyethylene film provides a bond break between the slab and the plywood subfloor underneath.

A radiant ceiling system is installed in much the same way as a radiant wall system.

An infrared thermograph of a hydronic radiant ceiling as it is warming up. The water flow is from left to right, as shown by the red and orange areas.

JAGA North America’s Low H20 panels provide the latest in contemporary looks and thermal performance. Small fans move air past the radiator fins, increasing low-temperature output up to 250%.

Contemporary radiant panel designs, like this one from Vasco Heating Concepts, can look like a work of art in themselves. Be sure to verify low-temperature performance when choosing any radiant wall panel.

This radiator from Runtal is part baseboard, part wall panel. Radiant panels come in many shapes and sizes to fit almost any application.

This home run distribution system is simple and effective, with a manifold accessible through a wall panel.

A panel radiator with an integrated thermostatic radiator valve

A panel radiator with an integrated thermostatic radiator valve (detail).

You can use renewably made hot water for your hydronic system—but designing for low water temperatures is critical to good performance.

Hydronic heating is the technology of moving heat using water. It has been used for decades in millions of North American homes, most of which have a gas-fired or oil-fired boiler as their hydronic heat source. Good hydronic design is also the “glue” that holds together renewable energy thermal systems that provide space heating and domestic hot water. In other words, pick a renewable heat source, do a good job with the underlying hydronics, and you’ll likely be pleased with the results. Treat the hydronics as “whatever,” and you’re likely to be disappointed.

In the past, solar hydronic heating meant using solar collectors as sunny-day substitutes for conventional boilers or water heaters. Designers focused on the collectors, storage, and control aspects of the solar subsystem, but devoted little thought to a compatible means of distributing solar-derived heat within the building.

Most hydronic distribution was designed around high-temperature supply water. Residential systems commonly used fin-tube baseboard heaters with water temperatures sometimes exceeding 200°F.

But those high water temperatures were beyond what solar collectors could produce consistently. Sure, there was an occasional “perfect solar day” in winter when the storage tank got hot enough to heat a home during the following night. However, performance over a typical northern heating season was often disappointing. As a result, after investing thousands of dollars in collectors, storage tanks, and controls, many early systems spent much of their time distributing heat generated by conventional fuels rather than by the sun.

The North American heating industry has a tendency to focus on heat sources rather than overall heating systems. This mindset continues to limit the performance of not only solar thermal, but also heating systems supplied by sources such as geothermal heat pumps and wood-fired boilers.

Low Temperatures = High Efficiency

All renewable heat sources yield better performance when combined with low-temperature distribution systems. To see why, take a look at the thermal performance characteristics of a solar collector and a geothermal water-to-water heat pump. The “Solar Collector” graph below shows how the thermal efficiency of a flat-plate solar collector is affected by the temperature of the fluid entering its absorber plate. On this typical sunny, midwinter day in the northern United States, the thermal efficiency of the collector drops rapidly with increasing inlet fluid temperature.

For example: If the fluid entering the collector is 90°F, the outdoor air temperature is 30°F, and the sun is bright (solar intensity is 250 Btu/hr./ft.2), the graph indicates that the fluid gathers about 56% of the solar energy striking the collector. However, if the entering fluid temperature is 160°F, while the other conditions remain unchanged, the collector’s efficiency falls to 33%—a significant “penalty” when the collector operates at the higher inlet temperature. It’s the result of greater heat loss from a collector to outside air, much like the increased heat loss associated with keeping your house at 75°F rather than 68°F.

The relationship between efficiency and the entering water’s temperature also holds true for hydronic heat pumps. The “Heat Pump” graph shows a similar effect for a modern water-to-water geothermal heat pump operating with a constant earth-loop inlet temperature (at the condenser side of the heat pump) of 45°F.

Comments (9)

With any renewable type system the rule should be make as much as you can whenever you can then use what you gain wisely, eg. do laundry when the sun is shining. The simple solution for moderating high temperatures especially with water systems is a mixing valve to only use what you need and leave the rest for other uses.Same with electricity.

I was looking at a small room heating idea using that under floor electric heat Mat stuff instead of a liquid based solution. How do these compare to one another in terms of cost, installation and efficiency?

The space is a small 12X13 bedroom I plan on building this summer in my walk out basement. The location is on the north side where it's going to get chilly.

I think Suzan's latest comments are spot on, but it depends on the specific situation and climate. It's almost always a balance between cost and benefit, resources and results.

In many climates (like mine), there isn't a lot of sun when space heat is needed, so focusing on solar space heating doesn't make a lot of sense. And when it does, passive solar design is usually a simpler and more cost effective option.

In general (again, specific homeowner goals and the specifics of the site, climate, and house will affect this greatly), I would focus first and foremost on efficiency, thermal and otherwise. Then I would look at efficiency of the heating system, with mini-split heat pumps being the current star performer in the cost/benefit arena. Then I would go after domestic water heating, using a solar hot water system. And then PV.

That said, recent drops in the cost of PV, and its simplicity compared to SHW, lead many people to go for PV sooner in their priority list. And each person has different goals, budget, patience for complexity, and attention span for RE and efficiency work. Many of my students and clients choose only to invest in PV, 'cause it's easy and effective, even after I've advised efficiency work first.

I think all work towards RE and efficiency is not a waste of time, and we each have different goals and situations. Active solar thermal _space heating_ is often of questionable cost effectiveness in my experience, in my moderate, cloudy-winter climate.

"Then I would look at efficiency of the heating system, with mini-split heat pumps being the current star performer in the cost/benefit arena."

Do you have some numbers to show this so that an apples to apples comparison can be done?? Can you point me at some spec sheets for some heat pumps and I can do my own analysis? (I'm an engineer)

I agree with the thermal efficiency statement, we still build lousy houses but the house you have is the house you have. A lot of articles (like this one) don't seem to take into account the retrofit market. I'm looking at the heating for my own home (presently baseboard electric) as my last electric bill was $600 (of course most of the charges were not for the actual electricity but that's another story).

"That said, recent drops in the cost of PV, and its simplicity compared to SHW, lead many people to go for PV sooner in their priority list."

In my area (Ontario, Canada), I would say this is because our government has made a large investment in solar programs (paying $0.83/kWH for solar when rate from utility is $0.08-0.10) and people jump on the bandwagon. From and engineering efficiency point of view solar still doesn't make a lot of sense (IMO). By the time the system is paid off (20 yrs), it's time for a new one (20 yr expected lifetime).

I don't have cost comparisons handy for heating systems, and I'm out of the country with poor connectivity at the moment too. But I encourage you to do more research on mini-split air-source heat pumps. The COPs claimed are in the 2-5+ range, which means 2 to 5+ times the heat for your kWh. And the cost is very modest -- in the $4-10K range installed complete, depending on home size and number of indoor units.

When I referred to the recent drops in PV, I was not talking about incentives at all, but the actual installed costs of systems, which has come down dramatically in the last several years, due to lower prices on PVs primarily. PV _very_ often makes purely financial sense with incentives, and it makes even more environmental sense. PV modules have _warranties_ of 20-25 years, and will be producing for 40+ years if well installed and maintained. I have modules on my roof that were installed in 1984 and are still going strong. Even if your very low prediction (20 years) were true (it is not), what else can you buy that is _productive_ and lasts that long? PVs are an amazing product that is very underrated.

I orginally got interested in solar because I live in Maine where the vast majority of people have a boiler fired by oil. It seems intellegent to use solar hot water. What I learned was regular baseboard was designed for 180 degree which is not realistic from solar in the winter. So I went about recommending everyone build a new home with radiant floors. I have now changed my mind. There is no logical reason to build an inefficient home. Proper air sealing and insulation is not very expensive. If you build right a simple air source heat pump, our favorite is Mistubishi mini split, is enough to heat the home. For really long term cold you may need a simple electric space heater, but those can be had for $30. Since energy efficient homes dont loose heat fast a day of cold temps will have little impact on the indoor temp even if the heat pump cannot produce. These days heat pumps run down to -17 degrees, that covers 99.9% of the days in Maine. Heat pumps are MUCH less expensive than solar powered radiant heat or low temp baseboard & radiators. The savings can go into air sealing and insulation. A small solar hot water system can meet domestic needs.

Suzan, You have come to the same conclusion that I often do -- thermal energy efficiency coupled with mini-split air-source heat pumps is the best option. In addition to the excellent reasons you cite, shifting the heating load to electricity also means that you may be able to power your heating system renewably, either through an on-site PV/wind/hydro system, through the renewable electricity your utility already sells, or through renewable energy credits.

Is there any benefit to using a water-to-water "geothermal" heat extraction device to remove heat from the fluid entering the solar thermal circuit? Would this, in your opinion, contribute to efficiency if the heat obtained via "geothermal" was used to augment heat gains obtained through the solar thermal loop?